The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Because of the excellent directivity, the power density of laser in the output direction is especially high, which greatly threatens human eyes. The wavelength of lasers used in the existing communication system is generally around 870 nm, 1310 nm and 1550 nm, which are all beyond the wavelength range of visible light. Because it is invisible, the light beam has a greater possibility to hurt human eyes. After the inventions of Erbium-Doped Fiber Amplifier (EDFA) and Wavelength Division Multiplexing (WDM) techniques, the output power of optical signals carried in fibers is even higher than that of conventional Synchronous Digital Hierarchy (SDH) devices. In addition, such apparatuses as RAMAN amplifier may be introduced into the existing communication system, which makes the power of the optical signals carried in fibers of some systems reach or even exceed 30 dBm. Such a high power of the optical signal greatly threatens the safety of operators and maintenance personnel of telecommunication system.
The safety level of laser apparatus and device is defined in Standard IEC60825, which specifies the specific operational measure and safety identifier as well. The International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) constitutes Standard G.664 for laser safety in communication system, and puts forward that an optical communication apparatus should be able to automatically reduce the output power to the safe power and even turn off the laser when a laser leakage occurs, which is expressed as the Automatic Power Reduction (APR) and the Automatic Power Shutdown (APSD) solution in G.664.
The APR is implemented by adding link status detection components and laser output control components in optical communication devices. When there is a potential laser leakage due to a failure on a fiber, the link status detection component in a downstream station detects loss of optical power, and determines that there is a potential laser leakage on an upstream link. The output power in an upstream direction of the downstream station is reduced via the corresponding laser output control component. The upstream station detects power reduction of the downstream station, and reduces output power in the downstream direction accordingly to reduce hazards caused by the laser leakage.
An APR procedure of a common optical amplifier in an optical communication system is described in detail as an example.
As shown in FIG. 1, when there is a potential laser leakage due to a failure on the west fiber from station B to station A, the west optical amplifier of station A detects loss of the optical signal sent from station B by the link state detecting component, and determines that there is a potential laser leakage on the fiber from station B to station A. The potential laser leakage may also exist on the fiber from station A to station B, and therefore, the west optical amplifier of station A instructs the east optical amplifier of station A to reduce the output power to make the laser leakage on the fiber from station A to station B compatible with the safety level constituted by Standard IEC60825, thereby protecting the operator and maintenance personnel. The east optical amplifier of station B detects the power reduction or loss of the east optical signal. The east optical amplifier of station B performs the same procedure described above. In other words, the east optical amplifier of station B notifies the west optical amplifier of station B to reduce the output power to make the laser leakage from station A to station B compatible with the safety level constituted by Standard IEC60825.
The implementation procedure of APSD is similar to that of APR, and the difference is that the APSD solution is to directly shut down a laser or an optical amplifier, rather than reduce the output power to a preset power.
The APR and the APSD of the existing communication apparatuses are basically implemented by detecting optical power. In other words, when the optical power in the receiving direction is abnormal, optical power output in its reverse direction is reduced or shut down at once, so the laser leakage in the reverse direction is ensured to meet the requirement of safety standard, and an opposite side device is also instructed to perform the protection procedure. The key of the APR and the APSD solutions is detecting whether the received optical power is below a certain threshold to decide whether there is a potential laser leakage on the fiber. However, the APR is unavailable with the optical power detection alone where RAMAN amplifiers or remotely-pumped amplifiers are applied.
As shown in FIG. 2, where a RAMAN amplifier or a remotely-pumped amplifier is applied, pump light of the RAMAN amplifier or the remotely-pumped amplifier is usually inputted into a transmission fiber in the reverse direction of signal light. When a west fiber from station B to station A fails, e.g. breaks, and poses a potential laser leakage, a west receiving side of station A always detects high optical power because a part of inputted pump light is reflected back and also a reversed stimulated Raman radiation generated in the same direction of the optical signals is received by station A. In such case, the optical power detection is insufficient for determining whether a network breaks and whether there is a potential laser leakage in a network correctly.
Multiple technical solutions have emerged to solve safety problems of laser leakage in optical communication systems where auxiliary amplifier devices such as RAMAN amplifiers or remotely-pumped amplifiers are applied. Typical solutions include detecting fiber line faults with an optical monitor channel, judging fiber line faults by a majority voting mechanism of optical channels, and even judging fiber line faults according to an Optical Signal to Noise Ratio (OSNR) and an electronic Signal to Noise Ratio (SNR).
Yet the safety problems of laser leakage include not only initiating the APR or APSD procedure when there is a potential laser leakage, but also launching a restart procedure safely to restore the communication system to a normal working status when the fiber line fault is repaired. The restoration procedure of an optical communication system is mentioned in the ITU-T G.664, illustrated as follows.
When a fiber link fault occurs, upstream stations and downstream stations initiate restoration timing respectively, and launch a self restoration procedure when the restoration timing is up. Provided an upstream terminal or a downstream terminal ends the restoration timing first, the terminal that ends the restoration timing first sends a detection pulse. When detecting the detection pulse, an amplifier in a direction of the detection pulse determines that an upstream link in the direction has been restored to normal, and sends the detection pulse along the same direction after amplifying the detection pulse. At last, the detection pulse reaches a downstream terminal in the direction. The downstream terminal in the direction, upon a receipt of the detection pulse, restores output power in a reverse direction. Likewise, the light in the reverse direction is also amplified by amplifiers along the reverse direction. If the fiber line has been restored to normal, the terminal which sends the detection pulse detects the light in the reverse direction and thus determines that the network has been restored to normal, and the whole restoration procedure is accomplished.
As shown in FIG. 3, provided that an upstream terminal initiates a restoration request first and sends a detection pulse at time point t1. The power of the detection pulse is lower than or equal to a safety power threshold. If a fiber link is restored, a downstream terminal detects the detection pulse and restores normal output power in a reverse fiber link at time point t2 after a time delay. The upstream terminal, upon detection of the output power in the reverse direction at time point t3, learns that the network has been restored, and restores the output power of the detection pulse to normal output power. Hence the whole fiber link is restored to normal. If the fiber link has not been restored, the downstream terminal is unable to detect the detection pulse and does not open the light path in the reverse direction. Consequently the upstream terminal receives no feedback and thus stops sending the detection pulse when a waiting time is up. If the reverse fiber link has not been restored, the optical output by the downstream terminal is unable to reach the upstream terminal, and the upstream terminal shuts down the optical output when a waiting time is up. Consequently the downstream terminal shuts down optical output because no optical signal is received.
The restoration procedure described in the ITU-T G.664 is performed between an upstream terminal and a downstream terminal. It is obvious that all amplifiers between the upstream terminal and the downstream terminal enter a failure state when one segment of the fiber line breaks. Because amplifiers adjacent to the broken segment shut down optical output once the fault is detected, which results in that even non-adjacent amplifiers consequentially shut down optical output. Because it takes time for each of the amplifier to restore normal output, hence a self restoration of the whole optical communication system needs a very long period of time which grows even longer when more cascade amplifiers are connected. Furthermore, once a fiber segment breaks, all neighboring amplifiers shut down output and all downstream amplifiers determine that the network fails and announce alarm because the downstream amplifiers receive no optical signal. As a result, all nodes in the network announce alarms, making it extremely difficult to locate the broken fiber segment in the whole network, and the abrupt alarms congest overhead channels and servers of the system, and the whole network is put in disturbance.